Interstellar Travel and the Fermi Paradox

chapter 12

Interstellar Travel and the Fermi Paradox

Provide ships or sails adapted to the heavenly breezes, and there will be some who will not fear even that void . . .

Johannes Kepler (in a letter to Galileo), 1593

Learning Goals

12.1The Challenge of Interstellar Travel

•How difficult is interstellar travel?

•Could we travel to the stars with existing rockets?

12.2Designing Spacecraft for Interstellar Travel

•How might we build interstellar spacecraft with “conventional”technology?

•How might we build spacecraft that could approach the speed of light?

•Are there ways around the light-speed limitation?

12.3THe Fermi Paradox

•Where is everybody?

•Would other civilizations really colonize the galaxy?

•What are possible solutions to the Fermi Paradox?

•What are the implications of the Fermi paradox to human civilization?

12.4The Process of Science in Action: Einstein’s Special Theory of Relativity

•What is “relative”about relativity?

•What evidence supports Einstein’s theory?

In an age of rapid technological progress, it may seem inevitable that our rockets will soon reach the depths of interstellar space. The reality, however, is that interstellar travel is much more challenging than bridging the distances to nearby moons and planets. There are engineering and physical constraints, not least of which is the cosmic speed limit—the speed of light. Nevertheless, we can envision at least some ways by which our descendents might someday travel among the stars.

The idea that humans might someday travel throughout the galaxy should make us wonder whether other civilizations have already achieved this ability. Indeed, if civilizations are common, it seems reasonable to expect that some societies—perhaps many—began colonizing the galaxy long before the earliest humans walked the Earth, and maybe even before Earth was born. This idea leads directly to the so-called Fermi paradox: If someone could have colonized the galaxy by now, why don’t we see any evidence of a galactic civilization?

We will begin this chapter by discussing both the challenges and possibilities of interstellar travel as we understand them today. Then, with that understanding in mind, we will confront the Fermi paradox, and see why, despite the seeming innocence of the question, its solution will undoubtedly have profound implications for the future of our own civilization.

12.1The Challenge of Interstellar Travel

Science fiction routinely portrays our descendants hurtling through the galaxy, wending their way from one star system to another as easily as we now travel from one country to the next. We have already used rockets to explore other worlds in our solar system. Could future generations to travel among the stars just by building larger versions of the rockets we use today? Perhaps surprisingly, the answer is no. The chemical rockets that have sent people to the Moon are wholly inadequate for taking people to the stars. We’ll see why in this section, and then in the next section we’ll see how we might move beyond the limits of currently rockets.

•How difficult is interstellar travel?

The fact that interstellar travel is a daunting enterprise is due to a simple circumstance: the tyranny of distance. The stars are so incredibly remote that only in the nineteenth century did astronomers develop instruments of sufficient precision to measure the distances of even the closest other suns. When it was realized just how far away these pinpoints of light are, the French philosopher Blaise Pascal was moved to write that “the eternal silence of infinite spaces” left him terrified.

Spacecraft Bound for the Stars

We’ve considered the vast distances to the stars in earlier chapters, using the scale model of the solar system introduced in Chapter 2. Here we consider the distances from the point of view of travel. Four of our past interplanetary probes—Pioneers 10 and 11, and Voyagers 1 and 2—are currently on their way out of the solar system; the New Horizons spacecraft will also head toward the stars after it passes by Pluto in 2015. How long will it take these spacecraft to reach the stars?

Let’s take the first, Pioneer 10, as an example. This spacecraft, launched in the early 1970s, took 21 months to reach its target, the planet Jupiter, before heading out of the solar system. This might seem speedy enough in view of the fact that the giant planet is never closer than 628 million kilometers to Earth. But our nearest stellar neighbor, the Alpha Centauri star system, is 70,000 times farther away than Jupiter. If Pioneer 10 were to cover the 4.4 light-years to Alpha Centauri at the same average speed at which it traveled to Jupiter, the journey would take 115,000 years. But Pioneer 10 was not aimed at Alpha Centauri or at any other deliberate target — its trajectory was designed to reach Jupiter and Saturn, not any particular stars beyond. If we plot its accidental trajectory along with the motions of nearby stars, we find that the closest Pioneer 10 will come to any star in the next million years is 3.3 light-years. In about 2 million years, the probe will reach the general neighborhood of the bright star Aldebaran, in the constellation Taurus.

You can now see why we did not equip Pioneer 10 or any of these other probes with instruments for studying planets in other star systems. Nevertheless, because the spacecraft themselves should survive unscathed for millions of years in the near-vacuum of interstellar space, we have included messages in case any extraterrestrial beings someday find them. The Pioneer probes each carry a small engraved plaque bearing a drawing of a man and a woman as well as diagrams giving the layout of the solar system and our general location in the galaxy (Figure 12.1). The Voyager craft, launched about 5 years after the Pioneer craft, carry a somewhat more sophisticated message consisting of pictures, multilingual greetings, and two dozen musical selections (ranging from Chuck Berry to Bach) on a gold-plated copper record (Figure 12.2). Although you might wonder how intelligible these earthly calling cards might be to any aliens, the chances that they will ever be found are slim. They are like bottled messages thrown into the ocean surf and were intended more as a statement to Earthlings than to extraterrestrials.

LIU Figure 12.2, p. 299. Please reduce to match size of next figure below. NOTE: These first two figures can be either placed in margin or laid out side-by-side as they are in TCP 4e, Figure 24.22, p. 729.

Figure 12.1 The Pioneer plaque, about the size of an automobile license plate. The human figures are shown in front of a drawing of the spacecraft to give them a sense of scale. The “prickly” graph to their left shows the Sun’s position relative to nearby pulsars, and Earth’s location around the Sun is shown below. Binary code indicates the pulsar periods; because pulsars slow with time, the periods will allow someone reading the plaque to determine when the spacecraft was launched.

LIU Figure 12.3, p. 299.

Figure 12.2Voyagers 1 and 2 carry a phonograph record—a 12-inch gold-plated copper disk containing music, greetings, and images from Earth.

Think About It . . . The Pioneer and Voyager “messages” are in the form of sounds, music, and pictures. But this assumes that any aliens finding these craft would have sensory organs similar to ours. Is it possible that our messages are too anthropocentric to even be recognized, or are there good reasons to think that E.T. will have eyes and ears, with characteristics similar to ours? Defend your opinion.

The Cosmic Speed Limit

The Pioneer 10 example makes the problem of interstellar travel quite clear. But it also seems to offer an obvious solution: Build spacecraft that can travel a lot faster. If it would take a little more than a hundred thousand years for Pioneer 10 to reach the nearest stars, then a spacecraft that travels 100,000 times faster should be able to make the trip in only a little over a year.

Unfortunately, this seemingly obvious solution is not allowed by the laws of physics. In particular, we know from Einstein’s special theory of relativity that it is impossible to travel through space faster than the speed of light. (We’ll discuss this theory and why it imposes a cosmic speed limit in Section 12.4.) This might not seem too limiting, given that light travels incredibly fast—about 300,000 kilometers per second (186,000 miles per second) — fast enough to reach the Moon in barely more than 1 second. But even at this remarkable speed, light takes time to travel the vast distances between the stars, which is why we measure stellar distances in light-years [Section 2.2]. The nearest star system, Alpha Centauri, is about 4.4 light-years away, which means it takes light 4.4 years to reach us from this system. Because that is the fastest possible speed of travel, any spacecraft we build would take longer than 4.4 years for the one-way trip and hence at least 8.8 years for a round-trip. To make a trip across our entire galaxy—a distance of 100,000 light-years—would take any spacecraft a minimum of 100,000 years.

Could it be that Einstein’s theory is wrong and that we will someday find a way to break this cosmic speed limit? This is unlikely. Special relativity merits the status of being a scientific theory because it is supported by an enormous body of evidence. Its predictions have been carefully tested and verified in countless experiments, so it cannot simply be “wrong.” While it might someday be replaced by a more comprehensive theory, the verified results will not simply disappear; the cosmic speed limit will almost certainly remain in place.

Energy Issues

Another challenge of interstellar travel is the tremendous amount of energy it would require, particularly if we wanted to send people and not just lightweight robotic probes to the stars.

Imagine that we wanted to colonize an extrasolar planet. To get a decent-size colony started, we’d need to send a fair number of people with many different sets of skills. For the sake of argument, suppose we wanted to send 5,000 people, meaning we would need a starship of similar capacity to the large starships used in the Star Trek television shows and movies. How much energy would such a ship require?

Interestingly, the minimum energy requirement doesn’t depend on the fuel source or the ship design at all. Sending a bowling ball flying through the air takes more energy than sending a baseball flying at the same speed, regardless of whether the energy comes from your arm, from a catapult, or from some kind of gas-powered launcher. Similarly, sending either ball flying at a faster speed takes more energy. That is, the energy required to put an object in motion depends on only two things: the object’s mass and the speed with which you want it to move.

We can estimate the mass of the starship by comparing it to other ships that transport large numbers of passengers. For example, the Titanic weighed about 18,000 kilograms per passenger, but its accommodations for most passengers were hardly roomy and it carried provisions for only a couple of weeks, not many years. If we conservatively adopt the same per-person weight for our starship, we would expect its total mass to be about 100 million kilograms. Let’s assume further that our starship travels at a modest 10% of light speed, which means it will take more than 40 years to reach the nearest stars. Now that we have estimated both the mass and the speed of our starship, a simple physics formula allows us to calculate the energy needed. (The required formula is the one used to compute kinetic energy, which is equal to 1/2 3m3v2, where m and v are the mass and velocity of the moving object.) The energy needed to get this ship to cruising speed is calculated to be 4.5 3 1022 joules, roughly equivalent to 100 times the world’s current annual energy use.

In fact, we should double this value, because the amount of energy required to slow down the ship for a soft landing at the colony is the same as that required to accelerate it to cruising speed. Thus, the total energy bill for the trip would be equivalent to at least two centuries’ worth of current world energy usage. We can put this in monetary terms. Using a typical price for home electricity (10¢ per kilowatt-hour), the energy cost of sending our craft to another star would be about $2,500,000,000,000,000,000. (To this you can add the cost of food and fresh towels for 40 years.) Clearly, unless and until we find a way to produce enormously more energy at vastly lower prices, large-scale interstellar travel will remain out of reach.

•Could we travel to the stars with existing rockets?

Today, when we think of space travel, we invariably think of rockets. But it wasn’t always so. The Greeks spoke of celestial travel in the mythological flights of Phaeton and Icarus, neither of which involved rockets and both of which ended badly. As described by the Roman writer Ovid (43 b.c.–18 a.d.), Phaeton was an impetuous youth who borrowed the Sun god’s chariot for a joy ride across the sky. Losing control of the horses on this ill-fated mission, Phaeton inadvertently started to set fire to the Earth. Humanity’s home was saved only when Zeus intervened by killing the cheeky charioteer. Icarus, the son of Daedalus, made a similar ill-advised move and became the first fictional flight victim. The feather wings Icarus used to cruise the heavens were held together with wax, and they underwent a fatal meltdown when, despite his father’s warnings, he flew too close to the Sun. These stories suggest that, while they were open to the possibility of other worlds, the Greeks regarded thoughts of human flight as hubris.

The Middle Ages saw a continuation of this less-than-enthusiastic view of human flight. Medieval thinkers were not intrigued by practical travel to the heavens, reserving such jaunts for spiritual ventures. Only after the Renaissance, when the philosophy of modern scientific inquiry took hold, did humankind once again consider the possibility of breaking Earth’s bonds. By 1687, Issac Newton had produced a treatise on universal mechanics that not only described the workings of the heavens but explained the physics required to reach them.

Newton’s third law of motion states that “for every action there is an opposite and equal reaction.” Envision the recoil of a gun when it is fired. The bullet moves in one direction, and the gun moves in the opposite direction. Squids, octopuses, and some other mollusks employ a similar technique. A squid takes in water that it then squirts out at higher speed behind it, thus propelling itself forward. A rocket operates slightly differently, vaporizing on-board fuel that is shot out the back. However, in both cases it is Newton’s third law that accounts for the forward motion.

Development of the Rocket

A rocket has been described as the simplest type of engine, and even Newton realized it could work in empty space. Serious thought about travel to other worlds began in the nineteenth century. In the 1860s and 1870s, the French author Jules Verne wrote influential stories describing travel to the Moon. His propulsion scheme used an oversize artillery shell specially constructed for the task. While this scheme was hardly practical (the enormous acceleration of the shell when fired would turn the passengers to pancakes), Verne’s writings stimulated investigation of space travel by three giants of early rocketry: Konstantin Tsiolkovsky (1857–1935) in Russia, Hermann Oberth (1894–1989) in Germany, and the American Robert Goddard (1882–1945). These fledgling rocket scientists explored many of the theoretical possibilities of this type of propulsion. In particular, they worked out the so-called rocket equation, which describes how a vehicle’s final speed depends on the propellant velocity (see Mathematical Insight 12.1.). Both Tsiolkovsky and Goddard realized that it would be difficult for a single rocket to reach escape velocity, the speed necessary to overcome gravity and leave Earth behind (about 11 km/s, or 25,000 mi/hr), so they proposed the use of multistage vehicles for space flight. The three pioneers also envisioned space stations, ICBMs, and ion drives.

At first, few people saw much benefit in turning these ideas into working hardware. In a 1920 technical publication, Goddard mused about the possibility that a sufficiently large rocket could reach the Moon. His speculation was immediately ridiculed by the New York Times, which claimed that no lunar-bound rocket could ever work since there is no air between Earth and the Moon. A rocket would have nothing to “push against.” However, contrary to the Times’ impression, rockets do not operate by pushing against air—or anything else. They simply employ Newton’s third law, firing hot gas in one direction so that the rocket moves in the opposite direction. In fact, atmospheres hinder the performance of rockets, because they create drag that slows rockets down.